Each ganglion cell gives off an axon that after a short distance divides into a peripheral process and a central process. The peripheral processes of the ganglion cells coalesce to form peripheral nerves. A purely sensory nerve will have only axons from such ganglion cells; however, mixed nerves, which innervate muscles, will contain both afferent (sensory) fibers and efferent (motor) fibers. At the target organ the peripheral process of an afferent axon divides repeatedly, with each terminal branch ending as a sensory receptor. In most cases the free nerve ending by itself forms a functional receptor, but in some the nerve ending is encapsulated by accessory cells and the entire structure (axon terminal plus accessory cells) forms the receptor.
The central axonal process of the ganglion cell either enters the spinal cord via a dorsal root or enters the brainstem via a cranial nerve. A central process typically gives rise to numerous branches that may synapse with a variety of cell types, including second-order neurons of the somatosensory pathways. The terminal location of these central branches varies depending on the type of information being transmitted. Some terminate at or near the segmental level of entry, whereas others project to brainstem nuclei.
Second-order neurons that are part of the pathway for the perception of somatosensory information project to specific thalamic nuclei where the third-order neurons reside. These neurons in turn project to the primary somatosensory cortex (S-I). Within the cortex, somatosensory information is processed in S-I and in numerous higher-order cortical areas. Somatosensory information is also transmitted by other second-order neurons to the cerebellum for use in its motor coordination function.
The organization of the somatosensory system is quite distinct from that of the other senses, which has both experimental and clinical implications. In particular, other sensory systems have their receptors localized to a single organ, where they are present at high density (e.g., the eye for the visual system). In contrast, somatosensory receptors are distributed throughout the body (and head).
Subdivisions of the Somatosensory System
The somatosensory system receives three broad categories of information based on the distribution of its receptors. Its exteroceptive division is responsible for providing information about contact of the skin with objects in the external world, and a variety of cutaneous mechanoceptive, nociceptive (pain), and thermal receptors are used for this purpose. Understanding this division will be the main focus of this chapter. The proprioceptive component provides information about body and limb position and movement and relies primarily on receptors found in joints, muscles, and tendons. The ascending central pathways that originate
CHAPTER 7  The Somatosensory System 109